The short answer: how to clean couch cushion foam is genuinely simple, and it is the wrong half of the job to worry about. Foam behaves like a sponge, so it takes water in readily and gives it up reluctantly, and a cushion that feels dry on the outside can hold moisture in the middle for days. Put a cover back on at that point and you have a warm, dark, damp space that grows mildew from the inside where you cannot see it. So the method is press the water out mechanically rather than waiting for it to evaporate, use a fan and airflow rather than heat, and test the middle with a dry paper towel before anything goes back on. Never put foam in a tumble dryer, which melts it.
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